1. Field of the Invention
The present invention relates to a micromotion mechanical structure which is used for example in a minute and high precision vibration-type sensor, an actuator for a high-performance robot, a recording or reading head for a magnetic or laser disc, a micro-optical system for optical scanning, modulation, etc., a micro-manipulator for use in a medical field, etc. and a gas precision control system.
2. Description of the Prior Art
Up to the present, micromotion mechanical structures used in the above technical fields have been produced mainly by a mechanical working technique such as lathe working, and owing to recent progress in the mechanical working technique, it is possible to produce a high precision product at a relatively low cost. However, owing to rapid progress in the field of microelectronics in these days, electronic parts have drastically been made smaller in size, and so further reduction of size has strongly been required for an entire system. It is however impossible to reduce a size of a mechanical element by any technique in extension of conventional mechanical working, as the recent rapid progress in electronics has made electronic parts smaller. It is therefore strongly expected to develop an innovative working technique which enables drastic reduction in size of a mechanical element.
For example, in the field of a magnetic or laser disc head, a detector portion of a head for reading a recording medium of high density is extremely minutely worked. Such detector portion is mounted on a metal arm to an extent of some centimeters for moving on the recording medium and is driven with the precision to an extent of 15-30 .mu.m by means of a servomechanism. Its drive pitch is limited by a natural frequency of its structure. As the size of a movable structure becomes smaller, its natural frequency becomes higher, and as a result, it becomes possible to drive the structure at a higher speed and therefore to operate the structure more minutely by combining it with a servomechanism. In the presently employed structure wherein the detector portion is mounted on a metal arm, however, it is very difficult to reduce the size of the entire structure, since even if its elemental parts were further minutely produced, it is very difficult to mount such very minute parts on the structure. On the other hand, the progress in the recording medium has enabled recording signals with a pitch to an extent of not larger than .mu.m Accordingly, it is understood that the largest problem for realizing high density signal recording is the large size of a head driving mechanism.
There is no report made on a technique which realize a minute structure of the head by any means different from extensions of the conventional technique.
Just recently, however, an innovative technique has been reported in connection with a silicon vibration type sensor. The present inventor has studied this report and found that this may be applicable to production of the minute head but there are still some difficulties to be overcome, as mentioned hereinafter.
The report was made by W. C. Tang, et al. in the Proceedings of IEEE Micro Electro Mechanical Systems (February 1989) at pages 53-59 entitled "Laterally Driven Polysilicon Resonant Microstructures". Attached FIG. 9 is recited therefrom and shows a plan view of a vibration-type sensor.
The structure shown in FIG. 9 is made of polysilicon deposited on a surface of a silicon substrate. In FIG. 9, fixing plates 13 to which fixed electrodes 11a and 11b are connected and supporting plates 14 to which folded beams 15 are connected are formed in contact with a silicon substrate 1. The fixed electrodes 11a and 11b and movable electrodes 12 which are connected to the folded beams 15 respectively are supported by the fixing plates 13 and the supporting plates 14 so that they are suspended on the silicon substrate 1. The fixed electrodes 11a and 11b and the movable electrodes 12 respectively are in the form of comb teeth and are interdigitated to each other to an extent of 1/3 of the teeth length. This vibration-type sensor has three pads for voltage supply, that is, pads 17 and 18 for supplying voltages to the fixed electrodes 11a and 11b respectively of reverse phases alternating between a supply voltage and the ground, and a pad 16 for a voltage always of the ground to the movable electrodes 12 through the supporting plates 14 and the folded beams 15. This means that when the pad 17 has a supply voltage, the pad 18 has a ground voltage and so the movable electrodes 12 are attracted toward the fixed electrode 11a by an electrostatic force and move in an up direction in the drawing. Subsequently when the voltage of the pad 17 is changed to the ground voltage, the voltage of the pad 18 is changed to a supply voltage and so the movable electrodes 12 are attracted toward the fixed electrode 11b and move in a down direction in the drawing. When the voltages of the pads 17 and 18 are changed in a cycle near to the natural frequency of the movable electrodes, the movable electrodes 12 vibrate in a large amplitude. Since the natural frequency of the movable electrode is a function of an atmospheric pressure, etc. on condition that the structure of the movable electrode is specified, it is possible to detect the pressure of air, etc., by detecting the natural frequency and so to use the structure as a sensor. By the way, the shapes of the folded beams 15 are changed by the movement of the movable electrodes 12 and owing to this stress the folded beams 15 tend to return the movable electrodes 12 to their original positions. Therefore, a moving distance of the movable electrodes 12 is a function of not only the applied voltage but also of the stiffness of the folded beams 15.
Such a vibration-type sensor made of polysilicon as above can be produced in a very minute scale. A process for the production thereof will be explained hereinafter with reference to the attached FIG. 10.
On one main surface of a silicon substrate 20, an oxide film 21 and a nitride film 22 are deposited and then a separation window 23 is patterned for separating a fixed electrode and a movable electrode [FIG. 10 (a)]. By depositing polysilicon and carrying out a patterning, a polysilicon electrode 25 connected to the pad 16 (shown in FIG. 9) and a polysilicon electrode 24 connected to the pad 17 or 18 (shown in FIG. 9) are formed [FIG. 10 (b)]. Then, a phosphosilicate glass (PSG) film 26 is deposited and patterned [FIG. 10 (c)], and a second polysilicon film 27 and a second PSG film 28 are deposited [FIG. 10 (d)]. The second PSG film 28 is patterned and by using it as a mask, the polysilicon film 27 is patterned and then the second PSG film 28 is removed [FIG. 10 (e)]. By immersing the obtained device in a hydrofluoric acid solution for a long time, the first PSG film 26 is removed to form the fixed electrodes 11a and 11b and the movable electrodes 12 (shown in FIG. 9) in the form suspended on the silicon substrate 20 by the second polysilicon film 27 [FIG. 10 (f)]. The thickness of the electrodes 11a, 11b and 12 is to an extent of 2 .mu.m. The numeral 13 in FIG. 10 (f) indicates the fixing plates 13 in FIG. 9.
On the other hand, in the field of optical modulation, an optical chopper has heretofore been prepared from a metal gear and an electromagnetic motor for driving the gear. Because high precision working is required, it is difficult to reduce a diameter of the gear less than some mm. Further, because the motor for driving the gear makes use of a coil, it is impossible to reduce a size of the motor to a minute level. Accordingly, a size of the optical chopper cannot be made smaller than some cm.sup.3. On the other hand, as is well known, it is possible in an optical or electronic integrated circuit to integrate almost all necessary functions on a small chip of some mm square. It is therefore understood that in this field large sizes of mechanical elements and a driving system therefor are the largest problems for realizing a minute entire system.
Recently, it was proposed to produce a joint for connecting movable mechanical parts on a surface of a silicon substrate by making use of a surface micromachining technique of polysilicon and in effect trial production was made for a gear, a spring, a slider and a micro-cutter. Particularly, L. S. Fan et al. made a report on "IC-Processed Electrostatic Micro-motors" in Technical Digest of International Electron Devices Meeting '88 (IEDM '88) at pages 666-669, wherein trial production of a polysilicon micromotor having a diameter to an extent of 100 .mu.m and a thickness to an extent of 1 .mu.m and its actual performance of revolution by electrostatic force to an extent of 500 rpm are described. This technique will be explained hereinafter with reference to attached FIGS. 25 and 26.
FIG. 25 (a) and FIG. 25 (b) respectively show plan view and a cross section along a line 25b-25b indicated in FIG. 25 (a) of a polysilicon step motor produced by L. S. Fan et al. and described in the above literature IEDM '88. This micromotor is constituted by three elements, that is, a rotor 201, a shaft 202 having a cap 204 to cover a center portion of the rotor 201 and prevent it from coming off and stators 203 located around the rotor 201 for applying an electrostatic force to the rotor 201. The shaft 202 and the stator 203 fixed to a fixing plate 207 are fixed to a silicon substrate 206 through an insulation film 205, but the rotor 201 is independent of the silicon substrate and can freely be rotated around the shaft 202. When voltages of opposite charges are applied to the rotor 201 and the stators 3 respectively, the rotor 201 is attracted to the stators 203 electrostatically. By applying voltages of the same phase to a couple of stators located oppositely with an angle of 180.degree. and rotating the phase sequentially as shown in FIG. 25 (a) by .o slashed.1, .o slashed.2 and .o slashed.3, the rotor 201 is rotated accordingly. It is possible to reverse the rotation direction of the rotor 201 by reversing the rotation direction of the phase to be applied to the stators 203.
The above polysilicon step motor can be produced to have a very minute size by a production process as explained hereinafter with reference to attached FIG. 26.
On one main surface of a silicon substrate 206, an insulation film 205 for example an oxide film or a nitride film is deposited, then on this insulation film 205 a first PSG film 211 and a first polysilicon film 210 are deposited and separation windows 208 for separating stators and a rotor are patterned in the films 210 and 211 [FIG. 26 (a)]. A second PSG film 212 is deposited and a window is opened in its center portion by patterning for a shaft [FIG. 26 (b)]. A second polysilicon film 213 is deposited and patterned to form a shaft 202 and a cap 204 [FIG. 26(c)]. The obtained device is immersed in a hydrofluoric acid solution for a long time to remove the PSG films 211 and 212 [FIG. 26 (d)], but the etching time is adjusted so that portions of the first PSG film are retained to form fixing plates 207. It is understood that from the first polysilicon film 210 the stators 203 and the rotor 201 are produced and the rotor 201 is produced to be independent of the silicon substrate 206.
As explained above, a mechanical structure made of polysilicon can be produced by a silicon IC production process and so its scale can be reduced to a very minute one. Further, mechanical elements of various different shapes can be formed on the same silicon substrate at one time by a patterning technique in the silicon IC production process and so it is not necessary to mount respective parts as in a conventional mechanical working process.
The present inventor has therefore studied the possibility of applying such technique generally to a micromotion mechanical structure and found that there are still some problems to be overcome, owing to the fact that the above technique makes use of a polysilicon thin film as a mechanical element. The problems mainly are as follows:
(1) Because a deposition rate of polysilicon deposition by sputtering is low, a time is required for producing a thick film. In an ordinary IC production process, the thickness of polysilicon film is to an extent of 1 .mu.m or less. It is of course possible to form a thicker film if a longer time deposition can be made. However in such case, an expensive apparatus is occupied for a longer time and so the production cost becomes higher. Moreover, very large internal stress is produced in a polysilicon thick film and causes bending or cracking in the substrate. In the technique as above mentioned, some portions of the polysilicon film are finally separated from the substrate and so the polysilicon structure is all the more easily deformed by the internal stress to cause a bend to an upper or lower direction and yield serious troubles of contacting with or sticking to the substrate. In effect, a number of such troubles have already been reported as to thickness to an extent of 1 .mu.m. For example, an article entitled "Microfabricated Structures for the Measurement of Mechanical Properties and Adhesion of Thin Films" in Digest of The 4th International Conference on Solid-State Sensors and Actuators (June 1987), pages 11-16, is referred to. Thus it is understood that it is very difficult to produce a polysilicon thick film having uniform internal stress.
(2) As mentioned in the above paragraph (1), it is in fact very difficult to produce a polysilicon thick film, but it is advantageous and so is required to make the polysilicon film as thick as possible for the following reasons:
In the above mentioned vibration-type sensor, the movable electrodes vibrate owing to electrostatic force yielded by a potential difference between the movable electrode and the fixed electrode. The electrostatic force is proportional to the cross sectional area of the opposing electrode surfaces. Accordingly, in order to obtain satisfactory electrostatic force in spite of that small cross sectional area (corresponding to the thickness to an extent of 1 .mu.m), it is necessary to apply a high voltage. In the above mentioned vibration-type sensor, the movable electrodes are moved in the vicinity of the natural frequency and so with relatively good efficiency. If movement at frequencies remote from the natural frequency should be made, however, it would be necessary to apply a voltage of about 200-350 V.
In the above mentioned step motor, the rotor is rotated owing to electrostatic force yielded by a potential difference between the rotor and the stators, and so the situation is similar to that of the above mentioned vibration-type sensor. In effect, it is necessary to apply a voltage of about 200-350 V in the above mentioned step motor.
Such voltage as above 200-350 V is extremely high in comparison with a voltage to an extent of 10 V used in an ordinary IC and so if the above mentioned structure should be introduced, an extra coil for the high voltage would become necessary in addition to an ordinary voltage source. Hence, the size of the entire apparatus becomes very large. Accordingly, if it becomes possible to increase the thickness of the movable and the fixed electrodes in the above mentioned vibration-type sensor or of the rotor and the stators in the above mentioned step motor, that is, to obtain a thick film of thickness for example to an extent of 10 .mu.m, the voltage to be applied can be reduced to 1/10 and so it is very advantageous.
(3) Up to the present, intensive research has been made on internal stress, mechanical constants, etc. of polysilicon, but the results show that they strongly depend upon process conditions when the polysilicon is formed. Therefore, in order to design a micromotion structure comprising polysilicon, a great number of data will have to be stored, but now not yet. At this stage, it is impracticable to precisely determine an optimum design of the structure before actual production.
As it is clear from the above explanation, these difficulties are inherent to a micromotion mechanical structure comprising polysilicon, and so it has strongly been expected that a new constitution of such structure which solves the difficulties and a production process which realizes the new constitution will be developed.